Zoologger: Female spider kills male to attract a mate

Species:Agelenopsis pennsylvanicaHabitat: northern US states, as far south as Tennessee and Kansas, hiding out in grassy meadows and lawns

Treat them mean, keep them keen? Female praying mantis and black widow spiders are notorious for their tendency to kill and eat males before, during or after sex. The behaviour is clearly risky, though – not least because the scent of a dead rival hardly encourages other males to try their luck.

Or so we thought. For male Pennsylvania grass spiders, the whiff of dead male seems to be exactly what they look for in a mate. They are far more likely to approach a female if she has recently killed and eaten a male.

Voracious appetite

It's not just insects that have reason to fear female Pennsylvania grass spiders. Males of the species can find themselves on the wrong end of a female's voracious appetite when the two meet to breed.

As mating strategies go, it seems a pretty foolhardy one: studies suggest females in urban settings are typically approached by no more than three – and as few as zero – males during their 3-week-long breeding season. Cannibalism seems to leave the females at risk of self-inflicted celibacy.

To work out what was going on, Jonathan Pruitt and his colleagues at the University of Pittsburgh in Pennsylvania captured young spiders from hedgerows and took them back to the lab.

The team selected 100 females and gave 50 of them a wounded male to kill and eat; the other 50 dined on a cricket. Some 10 to 24 days later, Pruitt's team gave 20 male spiders a choice: to approach the web of a cannibalistic female, or that of a female that hadn't consumed a male. Three-quarters of the males headed towards the cannibal.

"A history of cannibalism makes females even more attractive to subsequent suitors," says Pruitt.

Not only that, but the cannibalistic females were more likely than the other females to produce egg cases after mating – and their egg cases were more likely to hatch. "The benefits seem to overwhelm the costs," says Pruitt.

Sensible solution

Cannibalism makes sense simply because females are frequently larger than males, says Susan Riechert at the University of Tennessee, Knoxville, who wasn't involved with the study. "It is highly likely that some [males] will be viewed as prey," she says.

But that doesn't explain why males prefer to head towards a cannibal's web. Another of the team's observations could, however. They found that females typically ate only one male, even though subsequent encounters gave them opportunities to cannibalise again. Perhaps males are aware that females tend to cannibalise just once, and so feel more confident of their survival by approaching a known cannibal.

There are a few unanswered questions. For instance, why is a male spider a better dinner option for a breeding female than its standard prey? "Males seem to be some super-stellar multivitamins," says Pruitt – although Riechert thinks this is unlikely.

Then there's an even bigger question: if cannibalism is so advantageous, why do some females choose never to eat male spiders? Pruitt says it might be simply that some females can't afford to be picky. "Cannibalism may be particularly dangerous for unattractive females, since we predict their number of would-be suitors to be lower," he says.

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